1. Field of the Invention
The present invention relates to the well-known problem of generation of objectionable toxic or dangerous gases by equipment, particularly ozone gas generation by electrostatic imaging machines, and provides a convenient filter/catalytic converter unit which facilitates the simple cleaning or replacement of a dirt pre-filter element, as necessary, to protect the gas-neutralizing catalyst for the life of the machine.
As is well known, ozone gas is generated in electrostatographic copying equipment as a result of corona discharge during sensitization of the recording surface of the photosensitive member. This is also true of other stations within the copier which employ corona discharge devices. Ozone emissions into the surrounding machine environment are controlled by catalytic xe2x80x9cfilteringxe2x80x9d devices capable of conversion of the ozone to relatively harmless substances and are located in the copier exhaust stream. These filters are generally characterized as fixed-bed devices; that is, the catalyst is impregnated into a porous substrate which is integrated within the copier exhaust housing or is incorporated into the corona discharge electrode structure itself. Each type of ozone control system described hereinabove is limited in its ability to neutralize the ozone due to the physical constraints placed upon the catalysts containing element.
As will be appreciated, the amount of exposure of the ozone laden vapors to the catalyst determines the capacity and efficiency of ozone neutralization by the catalyst-containing element. Where the catalyst is entrained within or coated upon a porous or fibrous support, ozone laden vapors must be capable of penetration of this structure for contact with the catalyst. This presents problems since ozone laden exhaust produced by electrostatic copiers also contains numerous xe2x80x9cdirtxe2x80x9d particulates or contaminants (e.g. toner, paper fragments, etc.) These particulates can pass with the ozone laden exhaust to the catalyst support and, after a relatively brief period, impregnate the support material thereby diminishing the porosity of the catalyst- containing element. As the porosity of the support decreases, so too does the amount of catalyst accessible to the ozone-laden vapors. This can require frequent replacement of the catalyst-containing element or some sacrifice in the completeness of exhaust treatment in order to extend catalytic activity of the xe2x80x9cfilteringxe2x80x9d device.
These considerations also apply to the neutralizing of other objectionable, toxic and dangerous gases such as carbon monoxide, nitrogen oxides and similar gases by known catalyst beds whereby it is desirable to filter dirt contaminants from the discharge before it contacts the catalyst bed.
2. State of the Art
U.S. Pat. No. 4,143,118 discloses an electrographic apparatus incorporating a catalyst-impregnated filter screen associated with a corotron and heater element to achieve in-situ ozone pre-neutralization prior to the transmission of the exhaust gas to an ozone-neutralization unit having a granular catalyst bed filter. Back-flushing is required to remove entrapped particles from blocking the outlet end of the unit and creating a substantial pressure drop.
U.S. Pat. No. 4,315,837 discloses an electrographic apparatus incorporating a composite support matrix, such as glass beads, coated with ozone-neutralizing catalyst to form a filter element within the exhaust conduit of the apparatus. The catalyst matrix is held in place, at each end, by a foraminous member which permits passage of the exhaust gas and retention of the composite catalyst matrix without appreciable pressure drop thereacross. The foraminous member is in contact with the composite catalyst matrix, not easily separable therefrom for cleaning or replacement, and not indicated to be a dirt filter.
U.S. Pat. No. 4,388,274 discloses an electrographic copying machine incorporating an exhaust system for transporting the ozone-laden gas from each of the corona stations to an ozone-neutralizing filter unit containing a thin layer of ozone-decomposing catalyst retained between foraminous screens. The screens confine the catalyst bed and are not separable therefrom for cleaning or replacement nor are they disclosed to function as dirt filters.
The present invention provides a novel, convenient catalytic converter unit for pre-filtering dirt particles from a gas discharge containing an objectionable, neutralizable gas component, such as the ozone-laden gas discharge from electrographic copy machines in advance of the passage of the gas discharge through a gas-neutralizing catalyst bed, to exclude the passage of the dirt particles onto or into the catalyst bed where they can cause contamination and degradation of the catalyst, and plugging of the catalyst bed resulting in reduced gas-permability and the creation of a significant pressure drop thereacross which interferes with or substantially reduces the rate of gas flow through the unit.
The present catalytic converter units incorporate an easily-removable, cleanable and/or and replaceable dirt filter element having a dirt-trapping mesh which is upstream of, closely-spaced from, the catalyst bed and is supported for simple removal from the unit for periodic cleaning or replacement without interfering with the catalyst bed.